Metalloaluminophosphate molecular sieves contain a pore system that is essentially a network of uniform pores and empty cavities connected to one another inside a crystalline or semi-crystalline molecular structure. These pores and cavities catch molecules within the molecular structure. The molecules that are caught have a size equal to or less than the size of the pores and cavities. Molecules of a larger size cannot physically enter into the molecular structure.
The pores and cavities of metalloaluminophosphate molecular sieves are formed as a result of using template materials in the molecular sieve manufacturing process. During the formation of the molecular sieves themselves, a lattice type chemical structure is formed from the alumina, silica or phosphorous components that are used in making the metalloaluminophosphate molecular sieves. This lattice type structure essentially wraps around the template material, with the template material acting as a means of forming the pore structure within the molecular sieve. The resulting molecular sieve may be combined with other components for the benefit of adjusting various properties of the molecular sieve or to form larger particles.
To make the molecular sieve suitable for use, the template must be removed so that the pores and cavities are open to catch molecules, either for the purpose of adsorbing the molecules from the environment or to react the molecules to form a desired product. The reaction occurs when the molecules come into contact with catalytic sites located within the pore system, particularly within one or more of the empty cavities or cages as sometimes called.
The template is conventionally removed from the molecular sieve by calcining or burning out the template. An elution process can also be used to remove the template, although calcination is preferred. Once the template is removed, the molecular sieve is considered to be activated or ready for use. The activated molecular sieve has its pore system, including the empty cavities or cages open to the immediate environment, and ready for use.
Activated metalloaluminophosphate molecular sieves, particularly silicoaluminophosphate (SAPO) molecular sieves, that have catalytic sites within their microporous structure have been found to be especially sensitive to moisture. In general, significant exposure of the activated molecular sieves to moisture has been found to deactivate the catalytic activity of SAPO molecular sieves. Unfortunately, methods of protecting activated SAPO molecular sieves against the harmful effects of moisture are limited.
U.S. Pat. No. 6,316,683 B1 (Janssen et al.) discloses a method of protecting catalytic activity of a SAPO molecular sieve by shielding the internal active sites of the molecular sieve from contact with moisture. The template itself can serve as the shield, or an anhydrous blanket can serve as a shield for an activated sieve that does not include template. It is desirable to shield the active sites, because activated SAPO molecular sieves will exhibit a loss of catalytic activity when exposed to moisture.
U.S. Pat. No. 4,764,269 (Edwards et al.) discloses a method of protecting SAPO-37 catalyst from deactivating as a result of contact with moisture. The catalyst is maintained under storage conditions such that the organic template component of the molecular sieve is retained in the SAPO-37 molecular sieve, until such time as the catalyst is placed into a catalytic cracking unit. When the catalyst is exposed to the FCC reaction conditions, wherein the reactor is operated at 400° to 600° C. and the regenerator operated at about 600° to 850° C., the organic template is removed from the molecular sieve pore structure, and the catalyst becomes activated for the cracking of hydrocarbons. According to this procedure, there is little if any contact with moisture.
Mees et al., “Improvement of the Hydrothermal Stability of SAPO-34,” Chem. Commun., 2003, (1), 44-45, first published as an advance article on the web Nov. 22, 2002, discloses a method of protecting SAPO-34 molecular sieve, based on a reversible reaction of NH3 with acid sites of the sieve. The method transforms a H+-SAPO-34 into an NH4+-SAPO-34 in reversible way. The NH4+-SAPO-34 is said to be able to withstand severe steaming for an extended period of time without loss of structural integrity and acidity.
As new large scale, commercial production facilities that use metalloaluminophosphate molecular sieves in the production process continue to be implemented, protecting the molecular sieves from loss of catalytic activity as a result of contact with moisture continues to become an even greater challenge. Such challenge is particularly of interest in large scale commercial catalytic reaction systems, where the amount of molecular sieve material that is used is several orders of magnitude larger than typical pilot scale facilities. Such large scale systems will typically require at least 10 tons, and up to as much as 500 tons, of molecular sieve to operate efficiently. Accordingly, additional methods of protecting metalloaluminophosphate molecular sieves against loss of catalytic activity are desired.